Adjustable sieves



United States Patent ADJUSTABLE SIEVES Ernest Mare, Johannesburg, Unionof South Africa Application November 6, 1957, Serial No. 694,847 6Claims. (Cl. 183-9) This invention relates to screens for sizeseparation of solid particles or the removal of discrete particles ofsmall size carried in a gaseous current. It is particularly applicableto the screening of particles from air and is a continuation in part ofmy co-pending patent application 517,202, now abandoned. 7

It is the object of this invention to provide an eflicient vibratingscreen consisting of straight flexible filaments assembled in a frameside by side and parallel to one another under uniform tension anduniform distances apart with the spacing between the wires being amaximum of 50 microns.

In order to achieve these objects it is essential that the screenfilaments be smooth and of uniform cross section, that they beunsupported along their lengths for a distance at least nine inches andthat they be assembled under a high tension for example, between six andfifty tons per square inch in the case of metal wires.

In practice to achieve maximum uniformity of spacing the screens aremade of straight high tensile metal wires assembled side by side and insubstantial contact throughout their length, by which term is meant thatthe wires are assembled as closely in contact throughout their fulllengths as is practicable within the limitations imposed by minutevariations in diameter of even the highest grade wires and alsovariations of the wire diameters due to temperature changes. 7 Ascreenassembled in this manner will be substantially impervious to liquids andoifer excessive resistance to the passage of gases unless the wires areindividually vibrating so that the effectiveness of screening or inother words the effective widths of the spaces between the wires is forpractical purposes determined solely by the amplitude of vibration. Forany set of operating conditions, including velocity of the fluid stream,this is determined by the tension on the wires which is preferably madeadjustable.

- The invention is illustrated diagrammatically in the accompanyingdrawings in which:

Fig. l is a front elevation of the screen,

Fig. 2 is a sectional side elevation of one end of the screen showingthe method of tensioning the same.

Fig. 3 is a side elevation of the screen, and

Fig. 4 illustrates a modification.

As shown in Figures 1, 2 and 3 the screen consists of a strong and rigidframe 1 in which the parallel filaments 2 are mounted. The filaments mayfor example consist of heat and chemical resistant nickelchrome alloywires 3 of about 24 gauge, spaced in substantial contact with each otherand with each wire 3 under a tension of about 13 pounds. Generally thethinner the wires 3 the greater will be the efiiciency of the screen,but the thinness of the wires 3 is limited by practical manufacturingconsiderations, particularly as it is desirable that the ends of theWires 3 be connected together by brazing them to the bars 4, 5, toensure that in use there is no relative lateral displacement of thewires 3 and that the tension in all the ice wires is substantially thesame. Tension is applied to the wires 3 in any suitable manner such asby a fixed tensioning means or the adjustable tensioning deviceillustrated and comprising the tensioning bar 6 which is slotted toengage bar 4 and is pulled along the inclined surface 7 at one end 8 offrame 1 by means of the jack screws 9. When the wires 3 are at therequired tension the tensioning bar 6 is clamped in position by thebolts 10. A similar arrangement may be used at the end 11 of frame 1 butgenerally the bar 5 is merely clamped in a fixed position at this end. v

In the modification shown in Fig. 4 the bars 4, 5, are curved so thatthe wires 3 lie on a cylindrical surface and are parallel to the axis ofsaid surface. This construction may be used for building up acylindrical screen but with such a structure the vibration of the wirestowards the axis of the cylindrical structure will be very limited.

It will be seen from the drawings, especially Figs. 1 and 4, that theplurality of parallel filaments constituting the screen define asubstantially continuous surface from one end to the other and from oneside to the other.

In use the filaments 2 in the screens are vibrated normally by the flowof fluid onto them assisted where necessary by electrical, or mechanicalor sonic means, the screen becoming efiectively coarser the greater theamplitude of vibration. In any case under normal screening orclassifying conditions the screen due to its vibration will allow thepassage of particles of greater diameter than the normal width of thespaces between the filaments 2 when the screen is not vibrating.

Other factors being equal the amplitude of vibration will decrease withincrease in the tension of the filaments 2, but in practice the tensionon the filaments 2 must be adjusted to suit requirements when the screenhas been installed, since the efliciency of screening will depend onother factors such as temperature, humidity and the effectiveness of themeans causing vibration of the screen."

Under Wet screening of particles in a gaseous stream the tension appliedto the filaments is not so critical. For example, in screening dustladen air, water sprays 12 may be directed onto the upstream side of thescreen as shown in Figure 3. Due to the small width of the spacesbetween the filaments 2 and the surface tension of the water the latterappears largely to fill the spaces between the filaments, and entrap thedust particles. The water then flows in a turbulent manner down thedownstream side 13 of the screen with the entrapped dust particles andinto a reservoir forming part of an air duct in which the screen isinstalled leaving the air substantially free 7 of both dust and waterparticles.

Material which does not pass the screen falls on the upstream sidethereof but it is a peculiar and improperly understood feature of thescreens in accordance with this invention and clearly a contributoryfactor to the efficiency of the screens that under both dry and wetscreening condition that a very substantial collection of particlesoccurs on both the upstream and downstream side of the screen. Forexample under wet screening conditions generally about two thirds and inany case the major proportion of particles collected are discharged onthe downstream side of the screen. Under dry screening conditionsgenerally about one third and in any case a minor proportion of theparticles collected are discharged on the downstream side of the screen.A further unusual feature of the screen is that under dry screeningconditions it efiects an aggregation of the particles passing through itthe aggregates so formed being many times larger than the maximum sizeof particle in the gaseous stream being filtered.

For example efiicient screening of particles, under both wet and dryconditions, of less than one micron can be effected even though thewidth of the spaces between the filaments is about ten microns suchspacing often occurring due to imperfections in the wires even thoughthey are intended to be placed in substantial contact.

- Said screens are also found to be extremely effective under wetscreening conditions in effecting wetting of small particles carried ina gas stream.

These phenomena are directly related to the effective spacing betweenthe wires and are found to occur when the spacing between the unvibratedwires is less than 50 microns. Uniformity of spacing is essentialotherwise there will be lack of uniformity of screening over the wholearea of the screen and to achieve such uniformity in practice in themost convenient manner the Wires are placed in substantial contactthroughout their full length. For the same reasons uniformity oftensioning is essential and in the case of steel wires should be between6 and 50 tons per square inch of cross section of the wires.

A spacer such as the threaded rod 14 illustrated in Fig. 2 may besatisfactorily used as a spacer the threads being positioned betweenadjacent filaments 2.

It has been found that screens having an area of six square feet may beefiiciently operated with gaseous streams at rates varying between 750and 2,500 cubic feet per minute under wet conditions and 300 to 1,250cubic feet per minute under dry conditions. Normal rates are 1,000 and500 cubic feet per minute under wet and dry conditions respectively.

Where necessary the filaments 2 may be made of ferromagnetic materialand vibrated by an electro-magnet 15 as shown in Figure 3, or by amechanical vibrator 16 as illustrated in Figure l. The screen and framemay of course be vibrated as a whole by an electro-magnetically operatedvibrator of any normal type. Alternatively the filaments 2 may be madethrough resonance, to vibrate in sympathy with sound waves produced by asonic generator 17 as illustrated in Figure 2.

. With any form of vibrating mechanism the amplitude of vibration of thefilaments may further be controlled by attaching one or more lightbridges 18 across and in contact with them as indicated in Fig. 3.

To enable efifective vibration to occur however, the wires must belongitudinally unsupported by bridges or other means over a distance ofat least nine inches since supporting the wires at any lesser distanceapart will result in excessive pressure drop through the screen.

It will be seen, with a screen according to this invention, that it ispossible basically by variation of tension to obtain the minutevariations in the effective Widths of the spaces between the filamentswhich are necessary when dealing with particles of very small, size. Itwill also be seen that the screen is self cleaning or may be easilycleaned by brushing, or by a flow of a gas or liquid therethrough.

While the invention has been described as applied to the screening ofparticles from a gaseous stream using metal wire screens such wires mayunder certain conditions be replaced by filaments made of materialsother than metal, and the screens used for removal of solid particlesfrom a liquid stream.

What I claim as new and desire to secure by Letters Patent is:

1. A filtering screen comprising a pair of supports maintained in spacedapart relationship, a plurality of filaments assembled under tensionbetween said supports, said filaments being positioned parallel to onean other in a single layer and extending in the same direction, saidfilaments being uniformly very closely spaced together not more thanabout 50 microns apart and defining a substantially continuous surface,said screen being effective to remove very small particles of less thanabout 50 micron size from a fluid current, and means for adjusting thetension in said filaments.

2 A filtering screen as in claim 1 including means for flowing a liquidonto said filaments.

3. A screen as in claim 1 wherein said substantially continuous surfacedefined by said filaments is flat.

4. A screen as claimed in claim 1 wherein all of the filaments areconnected to said supports only adjacent the filament ends.

5. A screen as in claim 1 including means associated therewith forvibrating the filaments.

6. A screen as in claim 1 wherein the means for adjusting tensioncomprises a movable member connected to one end of said filaments and atleast two threaded members in threadwise engagement with said movablemember at spaced apart points and bearing against one of said supports,rotation of said threaded members be ing adapted to vary the position ofsaid movable member relative to said support to change the tension insaid filaments.

References Cited in the file of this patent UNITED STATES PATENTS

